Various abbreviations that appear in the specification and/or in the drawing figures are defined as follows:
3GPP 3rd generation partnership project
AP access point
BC broadcast mode
BS base station (e.g., Node B or e-Node B)
DF decode and forward
DL downlink
e- evolved (e.g., e-UTRAN or e-Node B)
IMT-A international mobile telephony-advanced
MAC multiple access mode
LDPC low density parity check codes
LTE long term evolution (3.9G)
MRS (M-RS) mobile RS
MS mobile station
OFDM orthogonal frequency division multiplex
RS relay station
Ss subscriber station
UE user equipment
UL uplink
UMTS universal mobile telecommunication system
UTRAN UMTS terrestrial radio access network
WiMAX world interoperability for microwave access (IEEE 802.16)
WLAN wireless local area network
A wireless relay network is a multi-hop system in which end nodes (UE/MS/SSs, referred to for convenience hereafter as UEs) are connected to the BS (e.g., base transceiver station, Node B or e-Node B) or AP (e.g. WLAN or WiMAX access point) via a RS. UL or DL traffic between UEs and the BS/AP may pass through and be processed by the RS. An example of one particular relay network concept is described in IEEE 802.16 Multi-hop Relay (MR), which is a recently established task group that is referred to as 802.16j. The MR effort is focused on defining a network system that uses RSs to extend the network coverage and/or enhance the system throughput.
A mobile multi-hop relay station, referred to herein as a RS, is useful for extending coverage and/or throughput of a BS. One non-limiting environment relative to these teachings is described in the context of IEEE 802.16 technology, also known as WiMAX. RSs are useful for extending coverage at a cell edge, to otherwise weak signal areas of a cell such as where shielded by a building, and for enabling stable communications on a moving platform such as a train. In that regard, the RS need not be fixed but may be mobile. The RS may be a dedicated RS as in the train example, or may be non-dedicated as in the case of another UE that is used to relay communications between a source mobile station and a destination such as the AP/BS. It is seen then that the RS may be embodied in a variety of hardware, fixed or mobile, dedicated to relay or not. To avoid inadvertent interference due to the addition of the RSs in the network, each node capable of acting as a RS is associated with a coverage area within which it will operate as a RS. Clearly, for a mobile RS that coverage area moves with the position of the RS itself.
While physical extension of the cell coverage area is the most visible function of RSs, they are also used to increase data throughput in the wireless network. Use of relays and co-operative diversity has been considered for the next generation of cellular systems by business and university researchers.
The main idea in using relays for increased throughput is that the source transmits data directly to destination in a conventional way and also transmits indirectly via a number of relays to the destination. While either of the UE and Node B can be a source or destination in any given communication depending upon whether it is UL or DL, the convention used herein by non-limiting example is that, where specified, the UE is acting as the source and the Node B is acting as the destination. At the Node B destination, joint decoding is typically required to combine received data signals from direct link (between the source and destination), and the indirect link (passing from the source to one or more relays then to the destination). The joint decoding at the destination is essential to achieve full co-operative diversity gain, but must be done with practical complexity.
Low Density Parity Codes (LDPC) have been considered in the context of co-operative diversity [see for example: A. Chakrabarti, A. de Baynast, A. Sabharwal, B. Aazhang, “LDPC CODE-DESIGN FOR THE RELAY CHANNEL”, IEEE Journal on Selected Areas in Communications, Vol. 25, No. 2, February 2007]. LDPC code performance may depend on the number of iterations and code length, which comes at the cost of higher complexity [see specifically P. Radosavljevic, A de Baynast, M. Karkooti, and J. R. Cavallaro, “MULTI-RATE HIGH-THROUGHPUT LDPC DECODER: TRADEOFF ANALYSIS BETWEEN DECODING THROUGHPUT AND AREA,” 17th IEEE International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC), September 2006]. At the destination, iterative joint decoding of signals received via the direct link and the indirect link may have relatively high complexity and hence increase the implementation cost of joint decoding significantly. A distributed coding scheme has been recently introduced that aims at using LDPC code components for the source-relay link and for the relay-destination link that perform equally well to maximize performance. [see A. Chakrabarti, B. Aazhang, “INFORMATION THEORETIC LIMITS AND CODE DESIGNS FOR COOPERATIVE COMMUNICATIONS” accepted for publication in IEEE Signal Processing Magazine, February 2007]. This requires joint coding design for LDPC code components, which the inventors see as potentially limiting the number of potential code designs and applications. Using large LDPC code lengths increases the complexity of iterative LDPC decoding in the relays and destination. This may prove problematic if large code lengths are used for best performance.
For example, full decoding in both the relay and the destination using a simple Hamming code was presented in a paper by A. Sendonaris, B. Aazhang, entitled: “USER COOPERATION DIVERSITY—PART I: SYSTEM DESCRIPTION” (IEEE Trans. Com, Vol. 51, No. 11, November 2003). The relay decodes the Hamming codeword iteratively and transmits the estimated corrected codeword with reliability for each symbol—i.e. soft symbols. At the destination, the received soft codeword is again iteratively decoded. This is a form of distributed iterative decoding. This is seen to be a very computationally expensive solution with practical limits to implementation.
What is needed is a way to exploit the advantages of joint decoding in a relay-based network without adding so much complexity that the joint decoding cannot keep up with real-time high data rates, which would render the overall system less practical. Embodiments of the invention detailed below presents such a solution, though as will be seen it is not limited to only real-time decoding and/or very high data rates.